I’m of the mind Artoo has the plans before the Tantive is captured. They certainly can’t be on the “ATM card” Leia is putting in him. I think all she’s doing at that point is recording the message and giving Artoo his marching orders.
Also keep in mind when they get to Yavin, they hook up Artoo to a large unit to do essentially a data dump. That looks like something that would take a little time to do.
Very good point and something I never thought about size of Data file. I think that the Plot was always Stolen Data Tapes and the Several Transmissions that were beamed aboard the Tantive IV.
Another contradiction is that the Size of the Death Star Plans were an issue in the original cut and having to beam them out. Come the Insert of the Darth Vader Slasher scene the Size of the File is no longer an issue and it can go from Data Tape to Simm Card. Another Contradiction to the Narrative there also.